A conventional air-to-air split system or packaged heat pump system is used to heat and cool buildings. Heating and cooling with this type of heat pump system is achieved by reversing the flow of refrigerant through the outdoor and indoor coils. In the heating mode it extracts heat from the outside air for space heating the interior of the building. In the heating mode the outdoor coil acts as a condensor and gives off heat to the air handling system for heating the building. The reverse action takes place in the cooling mode. The operation of the reversing valve is controlled by a heating and cooling thermostat inside the building. In conventional systems, a percentage of the air from the building is exhausted to the atmosphere. No attempt is made to recover heat from this exhaust air.